As a resin for a thermosetting powder coating composition, an epoxy resin, acrylic resin and polyester resin have been mainly used.
Also in order to improve weather resistance of general-purpose resins for powder coating compositions, there are proposals that a thermosetting fluorine-containing resin is used as a resin for a powder coating composition (JP-B-6-104792, JP-A-6-345822).
However when the thermosetting fluorine-containing resin powder coating composition is applied on a metallic substrate, there is no problem with weather resistance of the fluorine-containing resin coating film. However, for example, when the coating film is exposed outdoor for a long period of time, deterioration of the metallic substrate advances and corrosion thereof arises mainly due to rust. This phenomenon has not been recognized in case of general-purpose powder coating compositions being inferior in weather resistance and appears remarkably in case of a thermosetting fluorine-containing resin powder coating composition, particularly in case where the composition is coated by an electrostatic powder coating method.
The inventors of the present invention have found that when mixing a silicon oxide powder having a specific relation with a fluorine-containing resin powder, a thermosetting fluorine-containing resin powder coating composition being excellent in a property for protecting a substrate, particularly a metallic substrate can be provided.
It is known that since an expensive fluorine-containing resin is used for the thermosetting fluorine-containing resin powder coating composition, in order to increase a volume coating composition, powders such as a filler and silicon oxide as a pigment are blended to the composition (JP-B-6-104792, JP-A-6-345822 mentioned above). However a particle size thereof is relatively large, and an effect of protecting a substrate cannot be obtained as mentioned hereinafter.
Also there is a proposal that a silicon oxide powder having an average particle size of not more skin 50 μm (nm) is blended to general-purpose powder coating compositions (JP-A-51-42731). However in that publication, there is only a description that a flowability of the powder coating composition at fluidized bed coating and spray coating is improved but there is no recognition at all about not only thermosetting fluorine-containing resin powder coating composition but also a problem with the above-mentioned protection of a substrate.